The present invention pertains to use of bio-solid materials as a fuel or fuel additive in a cement making process. In particular, the present invention pertains to incorporating drying of the wet bio-solid materials in a conventional cement making process and cement making apparatus.
Bio-solids are defined as a residual by-products from biological activity or the residual by-products from processing of biological materials. Materials that are of interest for use in the method and apparatus of the present invention include sewage sludge, paper pulp residue, industrial sludges, food processing sludges and agricultural waste sludges.
Conventional cement making processes e.g. the manufacturing processes and apparatus for the manufacture of Portland Cement, are based upon the processing of limestone (CaCO2) by heating to achieve a cement clinker which is basically calcium oxide (CaO) chemically bound with other materials such as alumina, silica and iron.
In a conventional Portland Cement manufacturing process the main raw ingredient limestone is prepared with or without smaller amounts of materials containing alumina, silica and iron and are ground to produce what is called a raw meal. The meal is then conducted to a pyro-processing area, which may include preheaters and calciners to condition the raw meal for introduction into a rotary kiln where the intermediate product clinker is produced. The main kiln and the preheaters and calciners are conventionally heated with a burner using coal, oil or gas as the fuel component. The coal, oil or gas is generally mixed with preheated combustion air and ignited to provide the heat necessary to decarbonate and melt the raw meal to produce clinker. At the discharge end of the conventional rotary kiln, the hot clinker is introduced into a clinker cooler wherein large amounts of air are blown through the hot clinker to cool it to a temperature of about 200° F. After sufficient cooling the clinker can be ground into a final product. In the grinding operation of the cooled clinker a small amount of gypsum may be added to produce the finished Portland Cement.
One of the largest cost items in the manufacture of cement is the cost of fuel. With ever increasing prices for coal, oil and gas alternate fuel sources are constantly be sought for use in the process.
Among the materials which have been considered for use as alternate fuels are bio-solid materials, which have been processed prior to delivery to the cement manufacturing plant to remove moisture so that the dried bio-solid materials are of a moisture content and size to be introduced into the combustion processes for the cement manufacturing process.
U.S. Pat. No. 4,627,877 discloses a method and apparatus for continuously producing cement clinker. Patentees disclose use of waste material as fuel in a cement manufacturing process. The method and apparatus of the '877 patent provide for a cooling of cement clinker and the use of recovered energy to promote heat decomposition of combustible material.
U.S. Pat. No. 6,436,157 discloses a method for gasification of bio-sludge. The intracellular water of the bio-sludge material is reduced by heating the bio-sludge material at a temperature sufficient to weaken the bacterial cell walls. The water-reduced concentrated bio-sludge may then serve as a fuel source. Heating is accomplished by using a heat exchanger with details of the heat exchange process set forth at column 4, lines 47-67 of the '157 patent. The heating process is by a direct heat exchange, however, there is no disclosure of using indirect heat exchange with heat taken from a clinker cooler in a cement manufacturing process.
U.S. Pat. Nos. 5,895,213; 5,201,652; and 3,836,321 all describe various clinker cooling devices used to recover heat energy from the cement clinker being cooled. The heat recovered from the clinker cooler can be used in other parts of the process such as preheating.
U.S. Pat. No. 5,862,612 discloses a method and system for dewatering carboniferous materials using a vaportight pressure chamber. Water bound by capillary forces in fiber cells is reduced by a thermal mechanical dewatering process. Patentees describe a dewatering process involving three steps, the first of which is heating of the high porous organic material at an elevated temperature and at a high pressure thus reducing the moisture of the solid material. Patentees rely on high temperature and high pressure to effect dewatering of the material which may be used as a fuel. However, there is no disclosure of coupling this process with a cement manufacturing process.
U.S. Pat. No. 4,702,745 is directed to a process for dewatering high moisture content porous organic solids. Patentees process involves three steps, the first of which is heating the high moisture content porous organic solid at elevated temperature and high pressure to reduce the moisture content of the solid. Here again, there is no relationship of this process to a cement manufacturing process.
U.S. Pat. Nos. 1,965,513; 2,879,983; 4,285,140; 5,890,888; and 6,083,404 disclose methods for upgrading a material by drying using various techniques.